It looks like you're comparing 2 control names, likely in the lookup.
If you want to compare the values from these controls, you'll have to reference them more specifically.
For example, if it's text boxes, add .Text to the end of the control names, like so:
IfError(LookUp(DataTable1,'BMI Range_Column2'.Text='BMI output'.Text,'Price Per Area_Column2')),"$0")
If it's dropdowns, use .Selected, datepickers are .SelectedDate, ect.
Hope this helps!